Q:
I just watched a program on space which quoted that for every
grain of sand on the Earth there are a million stars in space.
Is this true, and if so, how do we know this?

A:
We can assume that the program was referring to the number of stars in
the observable universe -- not to be confused with the number of
observable stars, since astronomers can observe many distant galaxies
that contain billions of stars, but can't resolve the individual stars.
From the brightness of the galaxy, and knowing the brightness of an
average star at the distance of the galaxy, astronomers can estimate the
number of stars in a galaxy. The number of galaxies can be estimated
from deep surveys of selected regions of the sky. The latest estimate of
this type concludes that the observable universe contains about 70
sextillion stars, or 7 followed by 22 zeros.

A medium-sized grain of sand has a diameter of about 0.3 millimeters, so
a conical mountain of sand with a diameter twice as large as its height
would contain about the same number of stars in the observable universe
if it had a height of 10,000 meters.

"The Earth teaches us more about ourselves than all the books in the
world, because it is resistant to us." Antoine de Saint-Exupery Wind,
Sand and Stars, Penguin Books 1995